The present invention relates to a bone screw, a clamp for clamping the bone screw to a support element and an assembly of the clamp and bone screw. The present invention is for use in orthopedic surgery and particularly spinal surgery.
Surgical procedures are known in which a support element is fixed to the vertebrae in order to hold the vertebrae in a fixed position relative to each other and cause fusion of the spine. A variety of assemblies for connecting a vertebra to a support element for use in these methods of surgery are known. One family of devices are referred to as polyaxial screws. These devices generally comprise a bone screw which is inserted into a clamping element. The bone screw has a head portion with a convex surface that engages a corresponding concave surface within the clamping element. Relative movement between these two surfaces allows the angle of the axis of the screw to be moved relative to the axis of the clamp.
As a further development of these devices, screws which can be rotated within a single plane relative to a clamp have also been proposed. One such screw is discussed in EP-A-1774919. This describes a fixation element, or screw, which can be pivoted in a single plane relative to a receiving part. The movement of the screw in a single plane is limited by a pressure element. The pressure element engages the head of the screw to limit its movement. The head of the screw is formed from two opposite spherical surfaces joined by two cylindrically shaped surfaces. The interaction of the surfaces with the pressure element limit the screw to pivotal movement in a single plane.
US-A-2006/0155277 also relates to a screw that can be rotated within a single plane. An anchoring element, or screw, has a head comprising two flat guiding surfaces. The head is retained within an intermediate element which has corresponding counter surfaces to limit the head to rotate within a single plane.
A surgeon can select an appropriate type of screw depending on the range of relative movement required between the screw and the clamp. For example, depending on the situation in which the support element is required to be attached to a vertebrae, the surgeon may opt for a polyaxial screw which can be pivoted about several planes relative to a clamp, or a uniplanar screw, for example, as discussed in EP-A-1774919 or US-A-2006/0155277. In some circumstances, the surgeon may wish to use a screw which cannot be moved relative to the clamp. Thus, there is a requirement for a large inventory of suitable assemblies for fixing a support element to the vertebrae.
The majority of known polyaxial screw assemblies, including those described in EP-A-1774919 and US-A-2006/0155277, are also required to be assembled with the screw located in the clamp before the screw is inserted into a bone. This can cause difficulties when screwing the screw into the bone, particularly if the clamp is angled relative to the axis of the screw because in that case the clamp would need to rotate with the screw and will sweep out a cone as the screw is screwed in.
A clamp for a polyaxial screw is proposed in US-A-2003/0023240. The clamp can be inserted onto a spherical head of a polyaxial screw after the screw has been inserted into the bone. The clamp engages the spherical head by the use of a collet chuck comprising spring loaded tongues. The tongues can be deformed outwards to enable insertion or removal of a spherical head bone screw into the bottom of the clamp. However, this device cannot be adapted for use with screws other than polyaxial screws that can rotate relative to the clamp in several planes. It also presents a disadvantage because an axial force is required to be applied to both insert and remove the screw. This axial force may be required to be considerable to ensure that the spherical head is located securely in the clamp.
U.S. Pat. No. 6,716,214 relates to a polyaxial bone screw with spline capture connection. The polyaxial bone screw includes a bone implantable shank, a head and a retaining ring. The shank includes a capture structure with at least one spline with a wedged surface for engaging the retaining ring. In normal use the shank, head and retaining ring are assembled before the structure is screwed into the bone. In certain circumstances the shank can first be implanted before the capture structure engages the retaining ring. The use of wedge shaped splines means that subsequent disassembly is difficult.
WO-A-2004/041100 discusses a spinal stabilisation system using polyaxial bone fastener assemblies. The system includes bone fasteners with one or splines that engage a ring which is then inserted into a collar to provide polyaxial movement between the ring and the collar in a similar way to U.S. Pat. No. 6,716,214.